3. Flow Bifurcation How-to Guide

Flow Bifurcation is a mechanism which uses hardware capable Ethernet devices to split traffic between Linux user space and kernel space. Since it is a hardware assisted feature this approach can provide line rate processing capability. Other than KNI, the software is just required to enable device configuration, there is no need to take care of the packet movement during the traffic split. This can yield better performance with less CPU overhead.

The Flow Bifurcation splits the incoming data traffic to user space applications (such as DPDK applications) and/or kernel space programs (such as the Linux kernel stack). It can direct some traffic, for example data plane traffic, to DPDK, while directing some other traffic, for example control plane traffic, to the traditional Linux networking stack.

There are a number of technical options to achieve this. A typical example is to combine the technology of SR-IOV and packet classification filtering.

SR-IOV is a PCI standard that allows the same physical adapter to be split as multiple virtual functions. Each virtual function (VF) has separated queues with physical functions (PF). The network adapter will direct traffic to a virtual function with a matching destination MAC address. In a sense, SR-IOV has the capability for queue division.

Packet classification filtering is a hardware capability available on most network adapters. Filters can be configured to direct specific flows to a given receive queue by hardware. Different NICs may have different filter types to direct flows to a Virtual Function or a queue that belong to it.

In this way the Linux networking stack can receive specific traffic through the kernel driver while a DPDK application can receive specific traffic bypassing the Linux kernel by using drivers like VFIO or the DPDK igb_uio module.

Fig. 3.3 Flow Bifurcation Overview

3.1. Using Flow Bifurcation on Mellanox ConnectX

The Mellanox devices are natively bifurcated, so there is no need to split into SR-IOV PF/VF in order to get the flow bifurcation mechanism. The full device is already shared with the kernel driver.

The DPDK application can setup some flow steering rules, and let the rest go to the kernel stack. In order to define the filters strictly with flow rules, the Flow isolated mode can be configured.

There is no specific instructions to follow. The recommended reading is the Generic flow API (rte_flow) guide. Below is an example of testpmd commands for receiving VXLAN 42 in 4 queues of the DPDK port 0, while all other packets go to the kernel:

testpmd> flow isolate 0 true
testpmd> flow create 0 ingress pattern eth / ipv4 / udp / vxlan vni is 42 / end \
         actions rss queues 0 1 2 3 end / end